Visible light emission from Si/SiO2 superlattices in optical microcavities

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Abstract

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO2 superlattices. Placing them in a one-dimensional optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and top interfaces. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of a quartz substrate case. The addition of a broad-band high reflector to the superlattice surface results in enhanced narrow-band emission. These results show that a suitably designed planar microcavity can not only considerably increase the external efficiency of luminescence in Si/SiO2 superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.

Introduction

Efficient light emission from Si-based materials has been sought for some time, as such materials could be incorporated into the devices required by the opto-electronics industry using well-known Si processing technology 1, 2. The fabrication of Si-based light emitting materials has been a very active area of research recently 1, 3, stimulated in part by the discovery of intense visible light emission in porous Si (π-Si) formed by the electrochemical etching of Si 4, 5.

Room-temperature photoluminescence (PL) in the visible spectral region was recently observed in Si/SiO2 superlattices when thin amorphous Si (a-Si) layers were deposited in ultrahigh vacuum by molecular beam epitaxy (MBE) and the thin SiO2 layers were produced using a rate-limited ultraviolet ozone treatment [6]. These superlattices, deposited on (1 0 0) Si substrates, consisted of six periods of 1–3 nm a-Si and 1 nm of SiO2. The PL peak energy and intensity were found to change with the a-Si layer thickness indicating that quantum well confinement was occurring [6]. These PL results are significant as they indicate that electroluminescent or cathodoluminescent devices based on such a-Si/SiO2 superlattices should be achievable and this could lead to their use in optoelectronic and display applications. Owing to the small number of periods, the PL from the MBE-grown a-Si/SiO2 superlattice samples was quite weak. More recently we have described a method of fabricating a-Si/SiO2 superlattices with hundreds of periods using an automated deposition system [7]. The resulting structures, whether deposited on Si, quartz, or glass, have a bright PL at room temperature that is visible to the eye.

Here we report on the effect of placing such superlattices in a one-dimensional (planar) optical microcavity in order to enhance further their light emitting properties. Such optical cavities have recently been employed with some success in obtaining narrow band emission from π-Si [8]and in producing efficient π-Si light emitting diodes [9].

Section snippets

Experiment

An automated radio-frequency magnetron sputtering deposition system [10]was used to fabricate the a-Si/SiO2 superlattices. The typical deposition conditions were as follows. After a base pressure of 4–6×10−7 Torr was reached, argon gas was flowed in and the Si target was presputtered for 10 min. A Si layer was then deposited at a rate of ∼0.025 nm/s after which the substrate was rotated away from the Si target. Oxygen gas was then introduced into the chamber to create a sufficiently intense

Results and discussion

The PL obtained from a 425-period superlattice deposited on Si, quartz, and Al-coated glass substrates is shown in Fig. 1. For the Si and for the Al-coated substrates, there is a pronounced modulation of the PL intensity with a modulation period of 0.22±0.01 and 0.23±0.01 μm−1, respectively. The modulation period is consistent with optical interference of the emitted light in the optical cavity formed by the entire a-Si/SiO2 superlattice. Such optical interference features in the PL have been

Conclusions

The PL intensity enhancement obtained from optical microcavity formation is evident at all wavelengths across the visible spectrum. These results show that a suitably designed planar microcavity can not only considerably increase the efficiency of light emission in a-Si/SiO2 superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.

Acknowledgements

We thank L. Howe, G. Clarke, and J.-M. Baribeau for their help in the fabrication and characterization of the a-Si/SiO2 superlattices.

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